1. Field of the Invention
The invention relates to a former for use in a packaging machine for manufacturing and filling bags. This invention also relates to laminated gas barrier films.
2. Description of the Related Art
In preserving food or medicine during a relatively long time period, it is necessary to form packaging which can provide an excellent gas barrier properties capable of completely shutting out oxygen and water vapor (both of which can enhance decay and alteration of food or medicine) coming from the outside air. Packaging materials having excellent gas barrier properties for achieving this purpose include polypropylene and polyester films coated with a metal layer or a metal oxide layer, such as aluminum, alumina or silica. Another known materials which can be used for the same purpose are laminated gas barrier films formed by laminating together the above coated films and other films. All of these films are suitable for use in packaging snack confectionaries.
Conventionally, the aforesaid packaging is performed by using a bag-manufacturing/filling packaging machine which intermittently feeds out rolled films and transports them to a former which transforms the film into a bag.
However, with regard to the laminated gas barrier films made of commonly used packaging materials, in designing the thickness and the elastic modulus of these barrier films stresses acting on the barrier layers, which may cause crack formation in the barrier layer, are not taken into account. As a result, when bags are manufactured in the bag-manufacturing/filling packaging machine, the films passing through the top of the former will be greatly bent, and some cracks will thus occur in aluminum-coated layers (which are in fact barrier layers). This leads to a large reduction in gas barrier properties.
High barrier metallized OPP (Oriented Polypropylene) films are typically metallized to an optical density range of 2.0-2.4. This has been shown to be adequate to provide good flat sheet (non-elongated) barrier properties. However, such an optical density level has not been shown to provide good barrier durability during the bag forming process.
U.S. Pat. No. 5,698,317, entitled xe2x80x9cPolyolefin-based laminated filmxe2x80x9d, discloses the use of four layer packaging film having a polyolefin resin layer sandwiched between a polyolefin mixed resin layer comprising a petroleum or terpene resin and a heat sealable layer or non-sealable winding layer. A metal layer is then deposited on the surface of the polyolefin mixed resin layer. The metal layer is deposited following the discharge treatment of the polyolefin mixed resin layer. The disclosure of U.S. Pat. No. 5,698,317 is, in its entirety, incorporated herein by reference.
U.S. Pat. No. 4,297,187, entitled xe2x80x9cSurface treatment of plastic material,xe2x80x9d discloses the use of a discharge treatment method on a plastic surface in a controlled atmosphere of N2 and CO2. The disclosure of U.S. Pat. No. 4,297,187 is, in its entirety, incorporated herein by reference.
Commonly owned copending U.S. patent application Ser. No. 09/715,013 (International Publication No. WO 00206043), entitled xe2x80x9cBIAXIALLY ORIENTED POLYPROPYLENE METALIZED FILM FOR PACKAGINGxe2x80x9d, discloses the use of a high optical density aluminum layer with a specific structure of aluminum and aluminum purity. The disclosure of U.S. patent application Ser. No. 09/715,013 is, in its entirety, incorporated herein by reference.
Commonly owned copending U.S. patent application Ser. No. 60/357,837, entitled xe2x80x9cBIAXIALLY ORIENTED POLYPROPYLENE HIGH BARRIER METALIZED FILM FOR PACKAGINGxe2x80x9d discloses the use of a high crystalline polypropylene resin of 93-98% isotactic content with high optical density. The laminate film described in this application includes a high crystalline propylene homopolymer resin layer of greater than about 93% isotactic content having a first surface and a second surface, a polyolefin resin layer disposed on the first surface and having a discharge-treated surface, a metal layer having an optical density of at least about 2.6 deposited on the discharge-treated surface of the polyolefin resin layer, and a heat sealable layer or a winding layer disposed on the second surface. This laminate film has shown very good barrier properties.
This invention improves upon the moisture and gas barrier properties of the film as well as the durability of the metal layer.
During the bag manufacturing process, although there is usually a tensile stress of about 1-50 kgf/m2 acting on the films, such a stress may be set at a low level so as to reduce the maximum stress acting on the barrier layers. Further, although a typical bag manufacturing speed is 50-200 bags per minute (the length of each bag is about 150-400 mm), it is possible to avoid an excessive tensile force caused by a sudden impact during the intermittent feeding of the films. This can be achieved by using a roll assembly, setting a low tensile force, reducing the feeding speed for sending films for manufacturing bags, to reduce the tensile forces acting on the laminated films.
However, if the barrier property is to be improved by laminating together several specific material layers, it is necessary to provide an additional step for manufacturing gas barrier films, hence increasing the production cost. On the other hand, even if merely the rigidity of the films is improved, distortions will occur in the films when they are bent over the former. As a result, cracks will appear in the barrier layers, making it difficult to prevent deterioration of barrier properties during the bag manufacturing process. Furthermore, lowering the tensile force during the bag manufacturing process will cause zigzag and wrinkle formation in the films, while decreasing the film sending speed in the bag manufacturing process will cause a decrease in the productivity of the bag-manufacturing/filling process. Accordingly, none of these measures are effective.
The invention provides a former for use in a bag manufacturing and bag filling packaging machine. The former includes a cylinder, a collar for guiding a film into the cylinder and a former top. The minimum radius of curvature of the former top along a film guiding direction is equal to or larger than 0.27 mm, the maximum radius of curvature of the former top along the film guiding direction is equal to or smaller than 0.50 mm, the average surface roughness of the former top is less than 10 xcexcm, and the number of high projections per unit length of 10 mm of the former top along a width direction of the film guided by the collar into the cylinder is less than five. A high projection is defined as being a projection of the surface of the former top that is as tall as or taller than 5 times the average surface roughness of the former top.
The invention also provides a method for manufacturing a bag that includes providing a laminated film comprising a gas barrier layer and support layers disposed on both sides of the gas barrier layer and forming the laminated film into a bag by guiding the laminated film over a former top of a former. The minimum radius of curvature of the former top along a film guiding direction is between a critical radius and 0.5 mm. The critical radius is defined as being the radius of the curvature of the former top which causes an elastic limit strain at an innermost-portion of the support layer immediately outside the barrier layer when the laminated film is bent over the former top. Furthermore, the elastic limit strain is defined as being the minimum strain which causes the support layer to yield or break.
The invention further provides a laminated film to be fed in a film feeding direction, which includes a first support layer, a second-support layer and a gas barrier layer attached to a surface of the first support layer that is closer to the second support layer. The strain along the feeding direction at the innermost portion of the first support layer is less than an elastic limit strain along the feeding direction of the first support layer when the laminated film is bent so that the second support layer is inwardly folded and the minimum radius of the innermost portion of the bent second support layer is 0.2 mm.
The invention also provides a laminated film to be fed in a feeding direction, which includes a first polymer layer, a gas barrier layer disposed on the first polymer layer, a second polymer layer disposed on the gas barrier layer, and a third polymer layer. The following formula is satisfied:
(t1/t3)xe2x89xa7(68.4xc3x97t2+1.3)xc3x97(E1/E3)xe2x88x920.8
wherein t1 is the thickness of the first polymer layer, t2 is the thickness of the second polymer layer, t3 is the thickness of the third polymer layer, E1 is the elastic modulus of the first polymer layer along the feeding direction, and E3 is the elastic modulus of the third polymer layer along the feeding direction.